1. Technical Field
The invention relates to methods and systems for controlling transitions of a xe2x80x9clean burnxe2x80x9d internal combustion engine between lean and stoichiometric engine operating conditions.
2. Background Art
Generally, the operation of a vehicle""s internal combustion engine produces engine exhaust gas that includes a variety of constituents, including carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx). The rates at which the engine generates these constituents are dependent upon a variety of factors, such as engine operating speed and load, engine temperature, spark timing, and EGR. Moreover, such engines often generate increased levels of one or more exhaust gas constituents, such as NOx, when the engine is operated in a lean-burn cycle, i.e., when engine operation includes engine operating conditions characterized by a ratio of intake air to injected fuel that is greater than the stoichiometric air-fuel ratio (a xe2x80x9cleanxe2x80x9d engine operating condition), for example, to achieve greater vehicle fuel economy.
In order to control these vehicle tailpipe emissions, the prior art teaches vehicle exhaust treatment systems that employ one or more three-way catalysts, also referred to as emission control devices, in an exhaust passage to store and release select exhaust gas constituents, such as NOx, depending upon engine operating conditions. For example, U.S. Pat. No. 5,437,153 teaches an emission control device which stores exhaust gas NOx when the exhaust gas is lean, and releases previously-stored NOx when the exhaust gas is either stoichiometric or xe2x80x9crichxe2x80x9d of stoichiometric, i.e., when the ratio of intake air to injected fuel is at or below the stoichiometric air-fuel ratio. Such systems often employ open-loop control of device storage and release times (also respectively known as device xe2x80x9cfillxe2x80x9d and xe2x80x9cpurgexe2x80x9d times) so as to maximize the benefits of increased fuel efficiency obtained through lean engine operation without concomitantly increasing tailpipe emissions as the device becomes xe2x80x9cfilled.xe2x80x9d
The timing of each purge event must be controlled so that the device does not otherwise exceed its NOx storage capacity, because the selected exhaust gas constituent would then pass through the device and effect an undesired increase in tailpipe emissions. The frequency of the purge is preferably controlled to avoid the purging of only partially filled devices, due to the fuel penalty associated with the purge event""s enriched air-fuel mixture.
The prior art has recognized that the storage capacity of a given emission control device for a selected exhaust gas constituent is itself a function of many variables, including device temperature, device history, sulfation level, and the presence of any thermal damage to the device. Moreover, as the device approaches its maximum capacity, the prior art teaches that the incremental rate at which the device continues to store the selected exhaust gas constituent may begin to fall. Accordingly, U.S. Pat. No. 5,437,153 teaches use of a nominal NOx-storage capacity for its disclosed device which is significantly less than the actual NOx-storage capacity of the device, to thereby provide the device with a perfect instantaneous NOx-retaining efficiency, that is, so that the device is able to store all engine-generated NOx as long as the cumulative stored NOx remains below this nominal capacity. A purge event is scheduled to rejuvenate the device whenever accumulated estimates of engine-generated NOx reach the device""s nominal capacity.
Significantly, it has been observed that a gasoline-powered internal combustion engine is likely to generate increased levels of certain exhaust gas constituents, such as NOx, when transitioning between a lean operating condition and a stoichiometric operating condition. For example, such engines are likely to generate increased levels of NOx as each of its cylinders are operated with an air-fuel ratio in the range between about 18 and about 15. Such increased levels of generated NOx during lean-to-stoichiometric transitions are likely to precipitate increased tailpipe NOx emissions, particularly when the subject transition immediately precedes a scheduled purge event, because of the trap""s reduced instantaneous efficiency (i.e., the reduced instantaneous NOx-retention rate) and/or a lack of available NOx-storage capacity.
In response, U.S. Pat. No. 5,423,181 teaches a method for operating a lean-burn engine wherein the transition from a lean operating condition to operation about stoichiometry is characterized by a brief period during which the engine is operated with an enriched air-fuel mixture, i.e., using an air-fuel ratio that is rich of the stoichiometric air-fuel ratio. Under this approach, the excess hydrocarbons flowing through the trap as a result of this xe2x80x9crich pulsexe2x80x9d reduce excess NOx being simultaneously released from the trap, thereby lowering overall tailpipe NOx emissions which might otherwise result from the lean-to-stoichiometric transition.
The inventors herein have recognized that what is still needed, however, is a method of transitioning the engine between a lean operating condition and a stoichiometric operating condition that is itself characterized by reduced levels of a selected engine-generated exhaust gas constituent, such as NOx, whereby overall tailpipe emissions of a selected exhaust gas constituent may be advantageously further reduced.
In accordance with the invention, a method and system for transitioning an engine between a first operating condition and a second operating condition, wherein the first and second operating conditions are characterized by combustion, in each of a plurality of engine cylinders, of a supplied air-fuel mixture having a first and second air-fuel ratio, respectively, and wherein one of the first and second air-fuel ratios is significantly lean of a stoichiometric air-fuel ratio and the other of the first and second air-fuel ratios is an air-fuel ratio at or near stoichiometry (hereinafter xe2x80x9ca stoichiometric air-fuel ratioxe2x80x9d), the method comprising identifying at least two discrete sets of cylinders supplied with the air-fuel mixture at the first air-fuel ratio; and sequentially stepping the air-fuel ratio of the air-fuel mixture supplied to each set of cylinders from the first air-fuel ratio to the second air-fuel ratio, includes: identifying at least two discrete sets of cylinders operating at the first air-fuel ratio; and sequentially stepping the air-fuel ratio of the air-fuel mixture supplied to each set of cylinders between the first air-fuel ratio and the second air-fuel ratio. In this manner, the invention advantageously avoids operating any given cylinder in the range of air-fuel ratios likely to generate excessively large concentration of a selected exhaust gas constituent during such transitions from either a lean operating condition to a stoichiometric operating condition or a stoichiometric operating condition to a lean operating condition. By way of example only, where the selected constituent is NOx, the range of air-fuel ratios likely to generate an excessive concentration of NOx is between about 18 and the stoichiometric air-fuel ratio.
In accordance with another feature of the invention, in a preferred embodiment, torque fluctuations resulting from the use of different air-fuel mixtures in the several cylinders during transition are minimized by retarding the spark to any set of cylinders operating with a stoichiometric air-fuel ratio until all cylinders are operating at either the first or second operating condition. Thus, when transitioning from a lean operating condition to a stoichiometric operating condition, each set of cylinders is sequentially stepped between operating at a lean air-fuel ratio and operating at a stoichiometric air-fuel ratio, with spark being simultaneously retarded as to each set of cylinders whose respective air-fuel mixtures have been stepped to the stoichiometric air-fuel ratio. Similarly, when transitioning from a stoichiometric operating condition to a lean operating condition, spark is initially retarded to all sets of cylinders (each of which is operating, prior to the transition, with a stoichiometric air-fuel ratio). Then, as the air-fuel mixture supplied to each set of cylinders is stepped to the lean air-fuel ratio, the spark to those cylinders is simultaneously advanced.
In accordance with another feature of the invention, after spark has been retarded to all sets of cylinders transitioned from a lean operating condition to a stoichiometric operating condition, and with all cylinders operating at the stoichiometric air-fuel ratio, spark is preferably slowly advanced while air mass flow rate is decreased, either under the direction of an electronic throttle control or the vehicle driver. The spark and air-flow adjustment upon reaching stoichiometric operation in all cylinders ensures maximum fuel economy with little additional perceived torque fluctuation by vehicle occupants.
In accordance with another feature of the invention, where the invention is used in combination with a downstream device that stores a selected exhaust gas constituent, such as NOx, when the engine""s air-fuel ratio is lean and releases previously-stored selected constituent when the engine is operated at an air-fuel ratio at or rich of the stoichiometric air-fuel ratio, the method preferably includes enriching the air-fuel mixture to a third air-fuel mixture supplied to at least one cylinder for a predetermined time, whereupon the trap is purged of stored amounts of the selected constituent. In a preferred embodiment, the air-fuel mixture supplied to the last set of cylinders being stepped from a lean air-fuel ratio to a stoichiometric air-fuel ratio is, instead, immediately stepped to a rich air-fuel ratio to begin the purge event. Where desired, the air-fuel mixture supplied to at least one other set of cylinders, each already operating with a stoichiometric air-fuel ratio, is simultaneously stepped to the rich air-fuel ratio. Upon completion of the purge event, the enriched air-fuel mixture supplied to each enriched set of cylinders is returned, again in a xe2x80x9cstepxe2x80x9d fashion, to a stoichiometric air-fuel ratio.
The above object and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.